Inhibitor for aluminum in alkaline solutions

ABSTRACT

A METHOD OF INHIBITING THE CORROSION OF METALLIC ALUMINUM DURING ITS TREATMENT WITH ALKALINE SOLUTIONS. AS THE INHIBITOR IN THE ALKALINE SOLUTION, ACCORDING TO THE INVENTION POLYACRYLIC ACIDS HAVING A SPECIFIC VISCOSITY IN THE RANGE OF 0.07 TO 0.90, PREFERABLY 0.09 TO 0.20, ARE USED IN A PREFERRED AMOUNT OF 0.05 TO 0.5% BY WEIGHT, BASED ON THE WEIGHT OF THE ALKALINE SOLUTION. INSTEAD OF THE POLYACRYLIC ACIDS THEIR WATER-SOLUBLE ALKALI SALTS CAN BE USED. BY THE PRESENT INVENTION THE UNDESIRED SIDE EFFECTS OF ALKALI SILICATE INHIBITORS, SUCH AS INCRUSTATION OF THE ALUMINUM, CAN BE AVOIDED.

United States Patent 3,672,821 INHIBITOR FOR ALUMINUM IN ALKALINE SOLUTIONS Hans-Joachim Schlussler, Dnsseldorf-Holthausen, Germany, assignor to Henkel & Cic GmbH, Dusseldorf, Germany No Drawing. Continuation of abandoned application Ser. No. 768,516, Oct. 17, 1968. This application Feb. 16, 1971, Ser. No. 115,756

Int. Cl. C23f 11/06 US. Cl. 212.7 2 Claims ABSTRACT OF THE DISCLOSURE A method of inhibiting the corrosion of metallic aluminum during its treatment with alkaline solutions. As the inhibitor in the alkaline solution, according to the invention polyacrylic acids having a specific viscosity in the range of 0.07 to 0.90, preferably 0.09 to 0.20, are used in a preferred amount of 0.05 to 0.5% by weight, based on the weight of the alkaline solution. Instead of the polyacrylic acids their water-soluble alkali salts can be used.

By the present invention the undesired side effects of alkali silicate inhibitors, such as incrustation of the aluminum, can be avoided.

This invention relates to the inhibition of metallic aluminum in alkaline solutions by the action of polyacrylic acids and/or their water-soluble salts.

It has been known to inhibit aluminum metal in alkaline solutions by the action of oxidizing agents, e.g. permanganates or chromates, which are active in the alkaline range. However, the efiiciency of these oxidizing agents is limited and relatively high amounts thereof must be used. In view of the physiological labor conditions and reasons connected with waste-water purification, chromates cannot be used in practice at present any more. The use of waterglass as an inhibitor for aluminum has also been known. It yields good results if it is applied in the necessary high proportions.

It has been found that the addition of water glass in the necessary high proportions, to alkaline solutions frequently results in unpleasant side effects, for example in crustformation or incrustation on the parts or objects treated with said solutions, particularly if said parts or objects are after-treated with acid, in order to remove the excess of akali. Such incrustations do not react with solutions conventionally used for their removal. They can be removed with hydrofluoric acid only, the use of which, however, raises physiological problems.

It has now been found that these difficulties can be eliminated if--instead of water-glass-polyacrylic acids having a specific viscosity of 0.07 to 0.90 are added to the alkaline solutions as inhibitors for aluminum. It has been found preferable to use polyacrylic acids having a specific viscosity in the range of 0.90 to 0.20. The inhibiting effect decreases in general if the specific viscosity is higher than 0.2. In some cases, for example in pickling aluminum, a not too strong reduction of the attack on aluminum is desirable. The specific viscosity 1; of a substance is calculated from the viscosity of the solution according to the following formula:

viseosity of the solution viscosity of the solvent It has been found that the polyacrylic acids are particularly eflicient at relatively low inhibitor concentrations and preferably 0.05 to 0.05% by weight, based on the weight of the alkaline solution. Equal amounts of alkali silicate have no inhibiting eifect and they even intensify the corrosion of metal caused by the alkali and a satisfactory degree of inhibition can be attained in this case by essentially higher alkali silicate concentrations only.

Instead of the free polyacrylic acids, water-soluble salts thereof can also be used, particularly the sodium, potassium and ammonium salt.

The alkaline solutions used in the treatment of aluminum in carrying out the present invention, contain particularly sodium hydroxide or potassium hydroxide. However, other substances of strongly alkaline reaction, such as alkali carbonates and alkali phosphates can also be used, either alone or in mixture with alkali hydroxides.

It has been found that by using the above described polyacrylic acids and/or their water-soluble salts, a retardation of the attack by the alkaline solution on the aluminum metal takes place, which is particularly suitable for commercial practice. At the same time, crust formation or the formation of coatings, which cause difiiculties in the application of the hitherto used alkali silicate inhibitors, is avoided.

The following examples describe some embodiments of the invention, to which the invention is not limited.

EXAMPLE 1 Three aqueous solutions were prepared (a) from water and 1% by weight of NaOH;

(b) from water, 1% by weight of NaOH and 0.2% by weight of polyacrylic acid having a specific viscosity of 0.15;

(c) from water and 1% by weight of NaOH and 0.2% by weight of sodium-silicate, in which the mol-ratio SiO :Na O was 3.3:1.

In each said solutions (a), (b) and (c) a weighed 99.5% sheet aluminum of 1 mm. thickness was immersed and leftimmersed for 60 minutes. The test pieces were then removed from the solution, rinsed with water and weighed in order to determine the weight of aluminum removed from the test piece during said 60 minutes. The weight of aluminum removed during immersion from the test pieces amounted in grams, per square meter, and per minute in test (a), i.e. treatment with NaOH only, to 0.17 grain" in test (c), i.e. treatment with solution (c), to 0.26 gram in test (b), i.e. according to the invention, to 0.024

gram

If the treatment is carried out in the above described manner, but with solutions (a), (b), and (c), which contain now in each case 3% of NaOH in solution, the weight of aluminum removed during immersion from test pieces amounted in grams per square meter, and per minute In 3 series (a), (b), (c) of tests aluminum sheet pieces of 99.5% by weight of aluminum, having a thickness of 1 mm. and dimensions of 5X 10 cm., were exposed by immersion to the action of aqueous NaOH solutions of different NaOH concentrations at 65 C. for 15 minutes.

In series (a) the NaOH solution contained no inhibitor;

in series (b) the NaOH solution contained 0.2% by 7 weight (based on the weight of the solution) of polyacrylic acid having a specific viscosity of 0.15;'in series the NaOH solution contained 0.2% by weight (based on the weight of the solution) of water-glass of a moi-ratio of SiO =3.3:l. 'Subsequently'the test pieces were re moved from the solutions, rinsed and dried.

The following Table 1 shows the weight of the alumi num removed in each case from, the test piecesduring immersion under the above described conditions, at different NaOH concentrations. In using instead of polyacrylic acid a water-soluble alkali salt thereof, substantially equal resultsare obtained.

TABLE 1 Weight of aluminum removed in grams per square meter and per minute Aluminum sheet pieces of the dimensions 0.1x5x l0 7 cm. were exposed to the actionof aqueous solutions containing 1% by weight of NaOH and 0.1% by Weight of polyacrylic acid of varying specificviscosity- Said test pieces were immersedinsaid solutions of 20 C. for 60 minutes.

The following Table 2 shows theweight of aluminum removed in each case from the test pieces during immer sionunder the above described conditions, in dependence on the specific viscosity of :the 'polyacrylic acids used.

TABLE 2 Weight of aluminum removed in grams per square and per minute Specific viscosityof the polyacrylic acid:

It will be understood from the above that this invention is not limited to the specific details and conditions disclosed above and can be carried out with various modifications. Thus-as already mentioned above-in the alkaline solutions for the treatment of aluminum according to the invention, other substances of strong alkaline reaction may be used, alone or in mixture with alkali hydroxides. As examples of such solutions, the following are mentioned: an aqueous solution containing 0.1 to 10% by weight of Na cO an aqueous solution containmg 1 to 5% by weight of NaOH and 0.05 to 0.5% by Solution 0 i ment of aluminuminamanner substantially analogous to that described abovein the case of solutions, in which the alkali consists of alkali hydroxide. g "'The term' inhibitor is used herein to denote compounds or compositions having the effect of slowing down corrosion of aluminum in alkaline solution. The term polyacrylic acids isused herein to denote products obtainable by polymerization of acrylic acid weight of K 'PO Such solutions are used for the treat-.

(CH CHCOOH). The test described in the above Ex-- ample 'l'was carried out atordinary room temperature of about. 20 C. The percent and parts stated herein are by weight, if not otherwise stated.

- What is claimed is:

1. Aprocess for inhibiting the corrosion of metallic aluminum in the presence of a strong alkalinesolution selected from the group consistingof aqueous sodium sists of the addition of from 0.05 to 0.5% by weight, based'on theweight of said alkaline solution, to said solurange of from 0.09 to 0.90 and selected from the group consisting of: polyacrylicacids and water-soluble salts of said acidsselected from the group consisting of the sodium salt, the potassium salt and the ammonium salt, said i specific'viscosity being determined according to the formula viscosity of the solution if viscosity of the solvent where the viscosity is determined of a 0.7% solution of said polyacrylic acid in a2 N sodium hydroxide solution in water; r r

2. The process of claim 1 wherein said compound has a specific viscosity inthe range of frorn0.09 to 0.20.

References Cited UNITED STATES PATENTS I 7 2,653,860 9/1953 Meyes 25279.5 X 2,671,717 3/1954 Ferguson 252-79.5 X 2,783,200 2/1-957 Crum et a1. 252181 UX 2,829,114 4/1958 Hervert 212.5 X 2,942,955 6/1960 Hannah 25279.5 X 2,975,039 3/19 61 Elliott 25279.5 X 3,080,264 3/1963 Zimmie et al. 134-22 R 3,405,072 10/1968 Kinnavy 252-396 X FOREIGN PATENTS 666,119 7/1963 Canada 134-2 MORRIS O. WOLK, Primary Examiner B. S. RICHMAN, Assistant Examiner U.S. C1. X.R.

=hyd-roxide and. aqueous potassium hydroxide which 'contion of. a compound having a specific viscosity in the 

